This invention relates to novel polymer beads and ion exchange resins produced therefrom, and to processes for producing the polymer beads and resins in uniform sizes.
Polymer beads which are to be functionalized to provide ion exchange resins are conventionally prepared by polymerization in an aqueous medium of a mixture containing a monovinyl aromatic monomer and a polyvinyl aromatic monomer in an amount effective to crosslink the monovinyl aromatic monomer. Typically, a monomer mixture contains about 50-90% by weight of monovinyl aromatic monomer such as styrene, the balance being a polyvinyl aromatic monomer such as divinylbenzene. The polymerization results in polymer beads having a range of sizes. Although the size range is controllable to some extent by the polymerization conditions a substantial proportion of the beads remains non-uniform, being either larger or smaller in size than what is desired for many uses. Since the beads are crosslinked, they cannot be recycled to the polymerization stage. Accordingly, the fines and oversize beads result in lower useful bead counts and sometimes must be discarded, thus resulting in a substantial economic loss.
A copending, commonly assigned application, U.S. application Ser. No. 927,225, filed July 24, 1978 and incorporated herein by reference, describes the preparation of weak base anion exchange resins by reacting a secondary monoamine with thermally crosslinked vinylbenzyl chloride type polymer beads. The thermal crosslinking results in a low degree or crosslinking (up to about 20% but more usually about 1-5%) such that the beads swell to considerable magnitudes when admixed with a swelling solvent preliminary to amination. The resultant loss of dimensional stability prevents formation of useful strong base ion exchange resins but not weak base ion exchange resins since the secondary amines used to prepare weak base resins will crosslink the beads, thus stabilizing the beads, whereas the tertiary amines will not crosslink, resulting in a product having too low solids for ion exchange use. Hence, when it is desired to use the thermally crosslinked beads as starting materials in the preparation of strong base ion exchange resin, the beads must be modified to prevent undue swelling and consequent loss of dimensional stability.